The present invention relates generally to a high energy coaxial cable for use in pulsed high energy systems.
Coaxial cables have long been used in the communication field and to a limited extent in pulsed power applications. Traditionally, these cables are designed for continuous transmission of relatively low power electrical signals having very broad range of frequency content. Because of the desire to transmit such signals with high fidelity, cables are carefully designed for specific uniform cross-section dimension over their length. The resulting impedance eliminates electrical mismatch when load and source impedances match the designed inter-connecting cable impedance. In such applications, transmitted electrical signals generally utilize only a thin surface layer of the conductor because of their broad spectrum and high frequency content. As a result, conductor cross-section is not a primary concern, and matched cross-section areas between inner and outer conductors are not usually considered in the design. Additionally, the insulating material used between conductors is usually selected based on its dielectric rather than thermal properties. Polyethylene, foamed polymers, and air are most frequently used.
Typically, temperature of the conductor, temperature capability of the insulator, and strength of the assembly in resisting radial stress produced by electromagnetic forces acting to repel the current carrying conductors, are of little significance in such designs.
In electromagnetic launcher and other pulsed power research, power pulses up to several tens of milliseconds duration and peak current of hundreds to thousands of kiloamperes must be transmitted between the power source and electrical load. Traditionally, power transmission is accomplished using large cross-section, high strength, rigid metal conductors. Such inter-connects require clamping mechanisms to restrain electromagnetic forces, often must resist recoil forces from high mass acceleration, and usually require inter-connections specifically designed for each installation. These inter-connections often produce intense electromagnetic fields which interfere with electronic devices and induce strong currents into other conductors, such as diagnostic cables located in the near vicinity of the current transmission path. These systems also introduce secondary problems such as high inter-connection inductance and potentially hazardous exposed electrical components.
In some system designs, commercially available coaxial cables have been used successfully to transmit power pulses described above. These designs require large numbers of cables to overcome deficiencies such as small, non-uniform conductor cross-sections and relatively low melting temperature of insulating materials. At megampere current levels and in repetitively fired systems where heating buildup is additive, the large number of conventional cables needed for an installation makes such designs impractical.
The following United States patents relate to various designs for coaxial cable.
4,987,274--Miller et al. PA1 4,960,965--Redmon et al. PA1 4,847,448--Sato PA1 4,626,810--Nixon PA1 4,614,926--Reed et al. PA1 4,584,431--Tippie et al. PA1 4,346,253--Saito et al. PA1 4,340,773--Perreault PA1 4,332,976--Hawkins
In particular, the Miller et al. patent describes a coaxial cable with insulation comprised of 60-25% fluorpolymer that is fibrillatable, 40-75% ceramic filler, and a void volume. The preferred fluropolymer matrix disclosed is PTFE, and the preferred ceramic filler is fused amorphous silica powder. The Redmon et al. patent relates to a coaxial cable with a conventional metallic center conductor and conventional polyethylene as the dielectric material. The outer conductor is formed over the dielectric layer which acts as a mandrel. The outer conductor comprises emplaced, small diameter carbon fibers which are stabilized in place by an impregnating resin. The Sato patent describes a coaxial cable having a metal deposited tape wound over the laterally wound shielding layer, which is, in turn, formed over an insulation layer about the conductor. The tape is disposed such that the metal layer is in contact with the laterally wound shielding layer. The Nixon patent relates to a low attenuation high frequency coaxial cable in which the center conductor is wrapped with a plurality of layers of low density PTFE dielectric material. In addition, at least one layer of high density, unsintered PTFE dielectric material is tightly wrapped around the low density tape. The high density material is then sintered to form an envelope to hold the low density material in position. The outer conductor comprises longitudinally extending, parallel, adjacent electrically conductive wire strands, which are applied with a slight helical lay around the dielectric of the cable. The Reed et al. patent describes a high power coaxial cable comprising an inner conductor and an outer conductor with insulated fittings disposed between the inner and outer conductors. The fittings are disposed near opposite ends of the cable to maintain a desired spacing between the inner and outer conductors. One of the insulated fittings has a plurality of longitudinal holes therethrough. The fitting is formed in two like sections joined at right angles to one another along a substantially 45 degree interface, thereby defining a short 90 degree turn for the inner conductor near the end of the cable. The fitting sections are retained in position by a surrounding mounting block. The Tippie et al. patent relates to a high voltage coaxial cable in which a room temperature curable silicone elastomeric material is applied under pressure to the outer surface of the cable braid. The material is forced between the voids of the braid and adheres to the primary insulation material at the insulation/braid interface. The Saito et al patent describes a coaxial cable comprising inner and outer conductors each provided as a corrugated tube. The conductors are arranged coaxially with a thermoplastic resin insulating member therebetween. The insulating member is composed of a spiral rib joined to an outer insulating tube. The special rib is made of high density polythylene and the insulating tube of low density polythylene. The Perreault patent relates to a dielectric system for coaxial electrical conductors. The system separates an inner and outer conductor, and is composed of a first layer of cellular polyparabanic acid. This layer directly contacts and provides a continuous skin circumferentially surrounding the inner conductor along its length. A second layer, consisting of crosslinkable polymeric laquer, provides a continuous skin enclosing the first layer. The Hawkins patent describes a dielectric system for coaxial electrical conductors. The system separates an inner and outer conductor, and is composed of a first layer of braided high tensile strength polymeric fluorocarbon filaments. The filaments form an open weave and surround the inner conductor. Surrounding the filaments is a layer of cellular polyparabanic acid tape, which is helically wound along the length of the cable. A polymeric film circumferentially surrounds the two layers, and is in turn surrounded by a continuous layer of a crosslinkable polymeric lacquer.